1. Field of the Invention
This invention relates to the field of oscillating tools, and particularly to systems and methods for reducing the reaction forces induced in a machine frame by such tools.
2. Description of the Related Art
Many workpieces are machined by means of an oscillating tool which is mounted to a machine frame. The motion of the tool gives rise to reaction forces, which induce vibrations in the machine frame. These vibrations can be coupled back into the tool and adversely affect the surface of the workpiece.
These reaction forces can be particularly troublesome when machining workpieces which require smooth surfaces, such as spectacle lenses. The reaction forces induced while lathing or surface generating such lenses can produce surface artifacts which result in surface aberrations, or discontinuities, which must then be removed by secondary processes such as fining and polishing. These additional process steps, which must be performed on every workpiece produced, are both costly and time-consuming.
Many approaches have been taken to reduce reaction forces of this sort. For example, U.S. Pat. No. 5,959,427 to Watson describes the application of xe2x80x9creaction cancellation forcesxe2x80x9d which are applied through the center of gravity of a moving stage assembly. As described in Watson at column 3, lines 58-61, the system is arranged to apply a net reaction cancellation force which is xe2x80x9cequal in magnitude to the mass of the stage multiplied by the acceleration of the stage in the stage travel direction.xe2x80x9d
Unfortunately, the approach described in Watson may be unacceptable in some circumstances. For example, when tool motion is produced using coils driven with magnetic fields, eddy currents are generated which result in drag forces. These drag forces can also be coupled to the machine frame and result in unwanted artifacts on the machined surface. These eddy current drag forces are proportional to the velocity of the coil in the magnetic field; Watson""s system does nothing to counter these forces.
A system and method are presented for reducing reaction forces induced in a machine frame by an oscillating tool, which overcomes the problems noted above. The invention counters forces which arise from both the acceleration and the velocity of the oscillating tool.
Reaction forces are reduced by means of a counterforce assembly, which includes a weight mounted on one or more linear slides, which are in turn supported by low friction bearings mounted to the machine frame which supports the oscillating tool. The weight moves along an axis coaxial to that of the tool. The weight is driven with a driving means responsive to a drive signal which is derived from two signals: a first signal which is proportional to the acceleration of the oscillating tool, and a second signal which is directly proportional to the velocity of the tool. By properly adjusting the acceleration and velocity components of the drive signal, the magnitude of the reaction forces induced in the machine frame by the oscillating tool can be substantially reduced.
The counterforce assembly preferably includes a centering means which prevents the assembly from moving to either end of the linear slides. An accelerometer is preferably mounted to the machine frame to sense its vibration, with the accelerometer output used to adjust the counterforce assembly""s drive electronics to reduce vibration to a minimum.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.